Okay, so we've got regular pirates, Asian pirates and Steampunk pirates...how about space pirates?
...or just pirates that look kind of alieny, since spaceships would look out of place.
....Or the space pirates could work if they crash landed, and the pirates in the game are just like, "THEY FELL FROM THE SKY, 'TIS AIN'T NATURAL!!!! Well, apparently their captain be gone, but since they know they've got us outgunned, they don't come cheap."
And then to get them you have to build a crashed space ship. But then since they're aliens, they're more expensive (because they could zap you) and to mix up the usual formula, their defensive units have higher defense than their offensive units have offense...but it costs the defensive units being much more expensive.
....Or they could be a limited edition item.
We're not planning to add new Units for now.